Changes for Web developers

HTML

The HTML5 {{HTMLElement("data")}} element has been implemented ({{bug(839371)}}).

The range state of the {{HTMLElement("input")}} element (<input type="range">) has been implemented, currently set enabled on Nightly and Aurora channel ({{bug(841948)}}).

JavaScript

ES6 Arrow Function syntax has been implemented ({{bug(846406)}}).

DOM

Support for the multipart property on XMLHttpRequest and multipart/x-mixed-replace responses in XMLHttpRequest has been removed. This was a Gecko-only feature that was never standardized. Server-Sent Events, Web Sockets or inspecting responseText from progress events can be used instead.

CSS

Support for CSS Conditionals (cssxref("@supports") and domxref("CSS.supports")) has been enabled by default ({{bug("855455")}}).

Changes for add-on and Mozilla developers

The properties parameter has been removed from the {{ifmethod('nsITreeView','getCellProperties')}}, {{ifmethod('nsITreeView','getColumnProperties')}} and {{ifmethod('nsITreeView','getRowProperties')}} methods of {{interface('nsITreeView')}}. These methods should now return a string of space-separated property names. ({{bug('407956')}})